The goal of the present investigation is to elucidate the contribution of motor neurons in the elaboration of rhythmic motor patterns, such as respiration or locomotion, driven by central pattern generators (CPGs). While the mechanisms of operation of CPGs have been well investigated, less is known about how motor neurons transform their input into a behaviorally relevant fictive motor pattern. It is therefore important to determine whether motor neurons are simply passive input-output systems or whether they possess properties that allow them to shape the motor pattern. Using the leech heartbeat central pattern generator as a model system, I will pursue the following specific aims: Specific Aim 1: Characterize the intrinsic membrane properties and electrical coupling of heart motor neurons. I will use standard electrophysiological techniques and dye-coupling experiments to characterize intrinsic membrane properties and quantify the strength aoupling to living motor neurons when their endogenous input is blocked and then assess the activity patterns assumed by these neurons. Specific Aim 3: Resolve the discrepancy in activity patterns assumed by our current mathematical model of CPG control of heart motor neurons and the activity pattern of motor neurons in the living system. For this aim I will modify the simplified model motor neurons of the existing model by incorporating the experimental findings of the first two aims. Together, these specific aims will contribute to our understanding of how motor neurons contribute to ongoing rhythmic motor patterns. Furthermore, the insights generated here will help establish general principles for understanding the interplay between central pattern generators and the motor neurons responsible for elaborating their output. PUBLIC HEALTH RELEVANCE The predominant result of spinal cord injury (SCI) is the loss of motor control, particularly for rhythmic behaviors such as walking. Understanding the general principles of functional motor pattern generation and in particular the role of motor neurons in shaping these patterns may prove useful in the development of rehabilitative strategies and prostheses for SCI. 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